Water-based lubrication of niobium nitride

Water-based lubrication has attracted wide attention as an oil-free lubrication method owing to its greener and cleaner lubrication means.However,due to operating in the water environment,most moving parts would inevitably suffer from abrasion,rusting,and aging problems.Developing a novel solid-water composite system with ultra-low friction and wear will open new possibilities for innovative lubrication material research and development.Here,we first revealed the water-based lubrication behavior of a high-hardness niobium nitride coating(NbN).In a three-phase contact environment(water,air,and NbN),oxidation and hydrolytic reactions of NbN result in the formation of"colloidal solutions",containing Nb_(2)O_(5)colloidal particles between the tribo-pairs.Utilizing the double electric layer repulsion and weak shear action of the"colloidal solution",NbN achieves ultra-low friction and wear;the corresponding values are as low as 0.058 and 1.79×10^(-10)mm^(3)·N^(-1)·m^(-1),respectively.In addition,other VB transition metal nitrides(VB TMNs)exhibit the same low friction feature as NbN in the three-phase contact environment;the friction coefficients are even lower than those in an oil-based environment.The water-based lubrication of VB TMNs provides a new reliable scheme for optimizing solid-water composite lubrication systems without additives and is expected to be applied in environments with high humidity or insufficient water coverage.